Refrigeration systems having scroll compressors are used in various applications, for example to provide chilling of freeze barrels or cylinders in frozen product machines, such as frozen beverage dispensers. An evaporator of the refrigeration system is heat transfer coupled to a freeze barrel, and the refrigeration system chills the evaporator during a chilling cycle of the freeze barrel to chill and freeze product in the barrel. The refrigeration system is also operable to heat the evaporator during a defrost cycle of the barrel to warm and defrost product in the barrel.
During operation of a refrigeration system having an electric motor driven scroll compressor, for example a refrigeration system used in a frozen product machine, the motor driven compressor is cycled on and off. Normally, with an uninterrupted power supply and controlled on/off cycling of the motor driven compressor, the time between turning the compressor off and turning it on again is sufficient for refrigerant pressures in the system to generally equalize, so that when the motor is turned on again, it will rotate the compressor in its proper and intended direction of rotation. However, it can happen that if there is a brief and transient loss of electric power during a chilling cycle of the refrigeration system, during which chilling cycle the pressure of refrigerant on the high side of the scroll compressor is considerably greater than the pressure on the low or suction side, in the short period while electric power is off a flow of refrigerant from the high to the suction side of the scroll compressor can rotate the compressor and its drive motor backward. Should electric power be restored while the drive motor and compressor are rotating backward, the electric motor can be energized in a reverse direction of rotation, such that it then rotates the scroll compressor backward in a reverse mode opposite from its intended and proper direction of rotation. Should reversal of compressor operation occur, the pressure on its suction side will begin to rise above a known limit prescribed for normal operation of the compressor, with an attendant loss of refrigeration and eventual overheating and unexpected shutdown of the refrigeration system.